In order to improve various safety devices such as a seatbelt and an air bag mounted at a vehicle, the safety devices may be controlled depending on a weight of a passenger seated on a seat of the vehicle. For example, in a case where the passenger is seated on the seat without fastening the seatbelt, a warning is generally issued to indicate that the seatbelt is not fastened. In addition, according to the United States regulations, the air bag must be deployed at a time of an accident such as a collision when an adult passenger is seated on a front passenger seat. Further, in a case where a child restraint system (i.e., a child seat) is backwardly fastened to the passenger seat so that an infant faces a driver of the vehicle, an impact caused by the deployment of the air bag may be adversely affected to the infant. Therefore, in such state, the deployment of the air bag is prohibited according to the US regulations. The determination of the adult passenger is based on a weight of an adult female having a small physique. As for the determination of the infant, certain criteria are also specified. The accurate detection and determination of the passenger's weight are extremely important in view of safety.
An example of an apparatus for measuring a passenger's weight, i.e., a load applied to a support portion of a seat is disclosed in JP3904913B (hereinafter referred to as Reference 1). According to a load detection mechanism for a vehicle seat disclosed in Reference 1, load detecting means achieved by strain gauges are arranged at four portions, respectively, serving as the support portions, between lower rails attached to the seat and leg portions attached to a vehicle floor. Loads detected at the aforementioned four portions are added up to thereby obtain the load of the passenger. In addition, the load detection mechanism disclosed in Reference 1 is not required to precisely detect the load for determining whether or not the passenger is seated on the seat, and whether the passenger is the infant or the adult. Therefore, the load detecting means may be provided at only some of the support portions of the seat to detect a portion of the load. A seat weight measuring device disclosed in JP2001-150997A (hereinafter referred to as Reference 2), which includes three support portions at front left and right, and a rear center of the seat, includes a load sensor only at the rear center of the seat. Thus, the number of load sensors decreases, which results in a reduction of parts costs, assembling costs, and wiring costs.
According to the seat weight measuring device that detects a portion of the load disclosed in Reference 2, the cost reduction is achieved. However, because a detected load value may change depending on an inclination angle of the vehicle, the determination accuracy may decrease compared to the apparatus disclosed in Reference 1 that detects the full load applied to the seat. For example, a smaller ratio of the load of a passenger or luggage on the seat is received by the load sensor arranged at the rear of the seat when the vehicle is being driven on a downhill rather than on a horizontal surface. Thus, even when the passenger on the seat is an adult, it may be determined that an infant or a child is seated on the seat.
A need thus exists for a load detection device which is not susceptible to the drawback mentioned above.